AWS offers four primary pricing models for compute resources: On-Demand, Reserved Instances, Savings Plans, and Spot Instances. Each model is designed for different usage patterns and budget requirements. On-Demand provides pay-as-you-go flexibility, Reserved Instances and Savings Plans offer discounts for long-term commitments, and Spot Instances provide steep discounts for using spare capacity with the risk of interruptions.
Spot Instances can achieve up to 90% savings compared to On-Demand rates. However, they come with the risk of interruptions, so they are best suited for stateless or fault-tolerant workloads.
The main risk of using Spot Instances is that AWS can reclaim them at any time to fulfill rising On-Demand requests. You receive a 2-minute interruption notice before the instance is terminated. As of March 2024, only about 5% of spot instances were interrupted in the previous three months, and following best practices like diversification can reduce downtime risk.
Amazon ECS has built-in support for Spot Instances. It manages compute diversification across instance pools and automatically replaces interrupted Spot Instances. ECS integrates with AWS Auto Scaling Groups to handle replacement and supports automated spot instance draining for graceful shutdowns and rescheduling of tasks.
Mixing On-Demand and Spot Instances allows you to achieve significant cost savings while maintaining service availability. For example, overprovisioning with Spot capacity can reduce costs to just 5-10% more than your original On-Demand cost, while ensuring availability through dynamic capacity management.
Reserved Instances and Savings Plans provide significant discounts (often 20-58% off On-Demand prices) in exchange for committing to 1- or 3-year terms. They are ideal for steady or predictable workloads and can be combined with On-Demand and Spot for optimal cost efficiency.
Discounts vary by payment option and term length. For example, Compute Savings Plans and Convertible RIs offer 21-50% off for 1- or 3-year terms, with higher discounts for partial or all upfront payments. Standard RIs can reach up to 58% off with a 3-year all upfront commitment.
ECS integrates with AWS Auto Scaling Groups to automatically replace interrupted Spot Instances. It also supports automated draining, where tasks receive a SIGTERM and then a SIGKILL signal, allowing for graceful shutdown and rescheduling on available instances.
Using multiple Spot Instance pools increases diversity and reduces the risk of downtime due to interruptions. The more pools you use, the less likely you are to be affected by a single pool's capacity fluctuations.
With EC2, you control the instance type and pools for Spot Instances, generally achieving higher discounts. In Fargate mode, AWS manages the infrastructure and Spot selection, simplifying operations but with less control over instance types.
A comprehensive strategy that combines On-Demand, Spot, Reserved Instances, and Savings Plans can achieve up to a 50% reduction in overall compute costs, while balancing risk and availability for your workloads.
ECS can be configured to automatically place instances in a draining state when a Spot interruption notice is received. This triggers a SIGTERM signal to running tasks, followed by a SIGKILL after 30 seconds, allowing for graceful shutdown and deregistration from load balancers.
Best practices include diversifying across multiple Spot Instance pools, using fault-tolerant workloads, and configuring ECS Auto Scaling Groups for automatic replacement. Following these practices can reduce the impact of interruptions, which are already infrequent (about 5% as of March 2024).
When a Spot Instance is interrupted, ECS places the instance in draining mode, sends termination signals to running tasks, deregisters them from the load balancer, and attempts to reschedule them on available instances in the cluster.
AWS Auto Scaling Groups (ASG) work with ECS to manage the lifecycle of Spot Instances. When an interruption occurs, the ASG attempts to launch a replacement instance from another pool, maintaining desired capacity and availability.
You can specify the weight of each capacity type (On-Demand and Spot) in your Auto Scaling Group configuration. ECS and ASG will then provision instances according to these weights, allowing you to control the cost and availability balance.
EC2 Spot gives you control over instance types and pools, generally offering higher discounts but requiring more management. Fargate Spot is managed by AWS, which selects the underlying infrastructure, simplifying operations but with less control and potentially lower discounts.
Spot Instance pool prices and capacities fluctuate independently based on supply and demand in each Availability Zone and instance type. Diversifying across pools helps mitigate the risk of interruptions due to these fluctuations.
In a scenario where On-Demand, Spot, Reserved Instances, and Savings Plans are used in equal shares at maximum discount levels, you could achieve up to a 50% reduction in overall compute costs.
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John Jamie
Content Writer
May 10, 2024
Featured
This article explains how you can use Amazon ECS spot, savings plans and reserved instances to optimize cloud costs.
Compute in AWS is primarily offered with four purchasing options:
On-demand instances follow the standard pay-as-you-go model, where you are billed by the second. This does not require any long term commitments, free’s you from the complexities of planning and purchasing compute, and are best suited for highly available fluctuating workloads.
Reserved Instances and Savings Plans are pricing models, where you commit to long term compute usage (usually 1 or 3 years in a more-or-less flexible way), in exchange for significant discounts. RIs and Savings Plans are perfect for workloads with steady usage or to handle the base load of unpredictable workloads .
Spot instances constitute the spare compute capacity in AWS which are available at steep discounts of up to 90% compared to on-demand prices. These are best suited for stateless or fault-tolerant workloads as AWS can claim these instances back to serve rising demand.
The purchasing options provided by AWS, all rely on the same underlying EC2 infrastructure which behave the same way. Spot is no exception.
Spot instances are the idle EC2 instances that are not being used to fulfill on-demand requests, and hence are made available at cheap prices with discounts of up to 90% off compared to on-demand prices. The prices of spot instances vary with time and demand.
All these price benefits come with a catch. Spot instances can be interrupted to fulfill rising on-demand requests. The instance will be given a 2 minute notice, after which, it is terminated. Fault-tolerant or stateless workloads hence work best with spot instances, as an interruption will have little to no impact on it.
AWS spot capacity is divided into spot instance pools. All spot instances of a specific instance type running in an AZ constitute a spot instance pool. For example: All C5.xlarge spot instances running in us-east-1a form a spot instance pool while all C5.2xlarge spot instances in the same AZ form another pool. Likewise, if you use the same instance type on three different AZ’s, you are consuming capacity from three different spot pools.
The prices and capacities of these pools fluctuate independent of each other. Tying this back to spot interruptions, the more instance pools you use, the more diverse you are, the less downtime you face, when demand increases. The “don't put all your eggs in one basket” concept applies to spot.
As of March 2024, with all the new capabilities we have, data shows us that spot interruptions have become fairly infrequent, with only 5% of spot instances interrupted in the last three months. The more we stick to best practices and proper diversification, the better spot as a whole will function.
ECS comes with built-in support for spot instances. Compute diversification across instance pools to automatic replacement of interrupted spot instances, are all taken care of by ECS.
A task in ECS can be launched in two ways:
In both these approaches, you can opt to make use of spot instances to bring in significant cost reductions, with EC2 spot generally providing higher discounts compared to Fargate. EC2 spot also requires you to choose the backing instance pools, whereas in Fargate, AWS takes the decision.
ECS automates spot lifecycle management by integrating with AWS Auto Scaling Groups. When there is an interruption, the ASG will try to provide you with a replacement instance, from another spot instance pool, depending on your configuration. For most fault-tolerant workloads, replacing an interrupted instance is more than enough to ensure availability.
ECS also supports automated spot instance draining. This can be enabled by passing a parameter to the ECS container agent via user data of your container instance.
Once enabled, ECS will place instances in a draining state when it receives the 2 minute spot interruption notice. All the tasks running on these instances will first be sent a SIGTERM signal, and then a SIGKILL signal 30 seconds after. This lets you stop your application gracefully, or even do that last mile log collection. ECS also deregisters all such tasks from the load balancer target group, while trying to reschedule them on the remaining available instances.
A mix of on-demand and spot capacity can bring in considerable savings while ensuring availability. For example: suppose you have a fleet running entirely on on-demand, which you want to overprovision by 50%. By using spot capacity to overprovision, you will only need to pay 5-10% more than your original cost.
With ECS, it is easy to achieve dynamic capacity type splits. You can specify the weight of each capacity type and the backing ASG handles the rest. For example: If you provide on-demand with a weight of two, spot with a weight of three, and you have 10 instances provisioned by the ASG. Then four of those instances will be on-demand and six of them will be spot.
Choosing Savings Plans and Reserved Instances can provide further gain. Savings Plans offer more flexible usage patterns than Reserved Instances. Below is a comparison of effective discounts relative to on demand prices for a range of purchase options.
Source: Mark Butcher via LinkedIn
Your final implementation should take all these options into consideration while keeping in mind their potential downsides. You can have a dynamic mix of on-demand and spot capacities wherein spot interruptions have minimal impact along with optimal multi-year commitments backed by proper understanding of your workload requirements.
In a very rough scenario of a mix of on-demand, spot, Reserved Instances, and Savings Plans in equal shares at maximum discount levels, you could achieve a 50% reduction in the overall cost.
Type | Compute SP for EC2 | EC2 Convertible RI | EC2 SP | EC2 Standard RI | ||||
|---|---|---|---|---|---|---|---|---|
1yr | 3yr | 1yr | 3yr | 1yr | 3yr | 1yr | 3yr | |
No upfront | 21% | 45% | 21% | 45% | 31% | 52% | 31% | 52% |
Partial upfront | 24% | 49% | 25% | 49% | 34% | 55% | 34% | 55% |
All upfront | 26% | 50% | 26% | 50% | 36% | 58% | 36% | 58% |
Discounts from On Demand Prices
